Means and method for noise suppression

ABSTRACT

The suppression of noise radiating from a high-velocity, unconfined stream of gas is accomplished by radially surrounding the unconfined stream of gas with a blanket of foam from a location adjacent its origin and extending axially of the stream of gas for a distance equal to several diameters of the stream at its origin.

United States Patent 11113,596,734

[72] Inventor MNYSJL [56] References Cited UNITED STATES PATENTS :53"? 3 2 1,348,828 8/1920 Fessenden 181/3302 51 Pmmd 5 3,047,267 7/1962 Peyrin 181/3302 [73] 3,208,552 9/1965 Seifert 181/33221 MuhmMw 3,270,835 9/1966 Kramer 181/33.221 3,442,350 5/1969 OBrien l8l/52X Primary Examiner-Robert S. Ward, Jr. [54] MEANS AND METHOD FOR NOlSE SUPPRESSION Anonwy-Beehler & Aram I0 China, 3 Drawing 1''.

18 52, 181/55 ABSTRACT: The suppression of noise radiating from a high- [Sl] Int. Cl. F01! 3/04, 'velocity, unconfined stream of gas is accomplished by radially 864d 33/06 surrounding the unconfined stream of gas with a blanket of [50] I'leld Search l8l/33, 43, foam from a location adjacent its origin and extending axially 5|,33.2l, 33 ;2 33.22l.}3.222,3 .223,11kt of the stream of gas for a distance equal to several diameters 42, 50, 52, 55; 239/261] I, 265.13, 265.17, 127.3 ofthe stream at its origin.

Patented Au 3, 1971 FIG 3 FlG.-2

MEANS AND METHOD FOR NOISE SUPPRESSION The present invention provides a noise suppressor which is adapted to surround a high-velocity stream of noise-radiating gas with a noise-attenuating foamed liquid.

Previously, considerable difficulty had been experienced in accomplishing .the muffling or suppression of the noise radiating from the exhaust of a jet engine. The high level of the jet engine noise near airports with major airline overhaul stations has become a serious health problem and a source of considerable annoyance to the residents of adjacent neighborhoods.

The disadvantages of prior devices have been overcome by the present invention which provides for surrounding a highvelocity, unconfined stream of gas, such as the exhaust from a jet engine, for a distance equal to several times the diameter of the stream of gas at its origin with an apparatus capable of supporting a blanket of noise-attenuating foamed liquid. Preferably, the inner wall of the blanket is spaced well away from the moving, high-velocity stream so as to prevent a rapid consumption of foam by direct physical contact with the moving stream. The maximum attenuation of noise is accomplished when the high-velocity stream of gas is surrounded by the blanket of liquid foam for an axial length at least equal to the diameter of the stream at its origin and preferably for an axial length greater than three times the diameter of the stream at its origin. The characteristics of the foam, such as shear strength, thermal stability, and particularly bubble size, are regulated to predetermined values so as to obtain the maximum attenuation of noise. In the drawings there is illustrated:

FIG. 1 is a perspective schematic view of a structure for holding a foam blanket radially spaced around a stream of noise-radiating gas;

FIG. 2 is a schematic front elevation of a self-contained noise suppressor; and

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2. Referring particularly to the drawings, there is illustrated a noise suppressor 10, in which a liquid foam blanket holder structure is provided. The liquid foam blanket holder structure includes outer foam barrier 12 which is generally cylindrical and composed of a liquid pervious material and a perforate inner foam barrier 14 which is mounted internally of and coaxially with outer foam barrier 12. An annular space 15 is defined between the inner wall of outer foam barrier 12 and the outer wall of perforate inner foam barrier 14. A support spider 16 is provided to support foam barriers l2 and 14 in operative coaxial relationship to one another. Support spider 16 is attached to and extends between outer support rings 18 and inner support rings 20. Support rings 18 and 20 define the configurations of foam barriers 12 and 14. Support rings 18 and 20 provide structural rigidity in combination with support spider 16 to the respective foam barriers 12 and 14. Outer foam barrier 12 is generally composed of perforate metallic screen or liquid pervious cloth material, and perforate inner foam barrier 14 is preferably composed of perforate metallic screen.

A foam-distributing manifold 22 is provided on noise suppressor 10 and is connected to foam discharge headers 24 so that foam is supplied to foam discharge headers 24 through foam-distributing manifold 22. Foam discharge headers 24 are positioned in operative association with annular space 15 so as to discharge and uniformly distribute foam 38 in annular space 15. A foam generator 26 is operatively connected to foam-distributing manifold 22 by means of manifold feed line 28. Foam generator 26 is adapted to supply a continuous stream of liquid foam to foam-distributing manifold 22 at a rate sufficient to replace any foam lost or consumed during the operation of noise suppressor 10. A tray 30 is mounted on carriage 34 in a position to receive and collect any liquid draining off of foam 38. Tray 30 is provided with a liquid return line 32 which conducts the liquid which collects in tray '30 into foam generator 26 for regeneration and recycle to 38 passes through liquid pervious outer foam barrier 12 and falls into tray 30 under the action of gravity. Carriage 34 is designed to be movable and adjustable as to height so that it can be easily moved to a desired location and adjusted to the required height to accommodate the requirements for noise suppression at various locations and at various heights above the supporting surface.

A jet engine exit cone 36 is illustrated particularly in FIGS. 2 and 3. The outlet of exit cone 36 is the origin of exhaust 35 which is discharged from exit cone 36 as an unconfined highvelocity, noise-radiating stream of gas. When noise suppressor 10 is positioned in operative association adjacent the origin of exhaust 35 at the outlet of exit cone 36, as illustrated particularly in FIG. 3, the exhaust 35 passes through passageway 37. Passageway 37 extends axially through noise suppressor 10 and is defined by the inner wall of perforate inner foam barrier 14.

The foam-generating system comprising the foam generator 26, the foam-distributing manifold 22, and the foam discharge headers 24 is preferably so regulated that the bubble size of the foam 38 is smaller than the perforations in inner foam barrier 14.

In operation the foam 38 is generally supplied continuously to the foam discharge headers 24 and extends slightly past perforate inner foam barrier 14 into passageway 37. The foam, in addition to attenuating the noise, serves to cool the structure and particularly perforate inner foam barrier 14.

The use of the term diameter throughout this specification and the appended claims is not intended to infer that the element to which it is applied has a circular cross section. The term diameter is used herein broadly to denote an average lateral dimension but not necessarily one that is constant throughout the periphery of the element to which this term is applied.

The noise suppressor of this invention needs only a minimum amount of structure to supply and maintain a noiseattenuating liquid foam blanket around an unconfined noiseradiating, high-velocity stream of gas. The perforate inner foam barrier 14 is preferably the minimum structure required to define passageway 37 axially through foam 38. The axial length of passageway 37 should be at least equal to the diameter of the unconfined high-velocity stream of gas at its origin and preferably at least five times this diameter. Passageway 37 may be shaped to fit the stream of gas as desired. If the stream of gas is expanding at a very rapid rate, the diameter of passageway 37 may be increased so that the walls of passageway 37 are expanded radially outwardly to accommodate the expanding stream of gas. Preferably, the diameter of passageway 37 is greater than the diameter of the stream of gas at least at its origin. When the stream of gas is an exhaust from the jet engine, the diameter of passageway 37 is at least equal to the diameter of the exhaust and may be several times the diameter of the exhaust. Preferably, the diameter of passageway 37 is from about two to three times the diameter of the exhaust at the outlet of the jet engine and is at least large enough to prevent the blanket of foam from being destroyed at a rate which is greater than that at which it can be generated.

The thickness of the liquid foam blanket should preferably be at least one-quarter the diameter of passageway 37, and in any event it should be thick enough so that it absorbs the majority of the acoustical power radiated to it.

The length of passageway 37 is established at such a value that it surrounds the high-velocity stream of gas for the period of time during which the majority of acoustic energy is radiated from the stream. This length is dependent upon the nature of the stream of gas and the total amount of acoustic energy which it is desired to absorb.

The foam-generating equipment is conventional in its design and operation and includes a water or other liquid supply, a foaming agent tank and metering device, and an air aspirator. The conventional components of the foam supply system are not illustrated in detail in the drawings.

The structure required to support the liquid foam blanket in operative association with the noise-radiating stream of gas is reduced to a bare minimum and is preferably composed primarily of deformable materials having little or no resilience. Using this structure, the maximum amount of radiated acoustical power is transmitted directly to the foam rather than to the supporting structure. Also, less foam is required to cool the supporting structure because it has a relatively small mass compared to the size of the foam blanket. The lightweight foam blanket-supporting structure is also economical to construct and maintain. The compactness and lightweight features of the foam blanket-supporting structure make it readily portable to and from various locations of use and convenient to store when not in use.

The foam used in the suppression of noise is preferably one having high strength and high density. The characteristics and properties of a wide variety of foams are well known. One suitable foam has been found to be that produced from an intimate admixture of protein and water. The bubble size of the foam is controlled principally by three factors, the nature of the foaming material, the pressure at which it is discharged, and the apparatus technique of aspiration. Procedures for the regulation of bubble size in foam are well known. It has been found that foam containing bubbles of a particular size will absorb a greater amount of radiated noise of a certain frequency than it will of radiated noise at other frequencies. There is a relationship between bubble size and frequency. It is possible to determine the optimum bubble size or range of sizes for a particular application by determining the amount of acoustical energy absorbed by the foam at different bubble sizes. This may be conveniently accomplished by measuring the increase or decrease in decibels as the bubble size in a liquid foam blanket surrounding a gas stream of constant acoustical output is varied.

The cleaning of the noise suppressor of this invention is very easy and simple to accomplish. Cleaning may be accomplished with clean water, and the water pervious nature of the foam barriers l2 and 14, respectively, renders it unnecessary to disassemble any part of the noise suppressor to accomplish complete cleaning. No toxic or corrosive special solvents need be used.

As will be understood by those skilled in the art, what has been described are preferred embodiments in which modifications and changes may be made without departing from the spirit and scope of the accompanying claims.

What I claim is:

1. A noise suppressor comprising:

an outer foam barrier;

an inner perforate foam barrier positioned within said outer barrier and spaced from the wall of said outer foam barrier;

a radially enclosed axially extending passageway defined by the inner wall of said inner barrier, said passageway extending axially completely through said noise suppressor; and

means for substantially filling the space between said inner and outer barriers with foamed liquid.

22. A noise suppressor of claim 1 wherein said inner and outer foam barriers are generally cylindrical and are mounted on a movable supporting structure.

3. A noise suppressor of claim 1 wherein the outer foam barrier is a liquid pervious cylindrical member and the perforate inner foam barrier is a cylindrical member positioned concentrically within said outer foam barrier, the means for substantially filling the annular space between said inner and outer barriers with foamed liquid including a foam discharge header adapted to discharge liquid foam uniformly throughout said annular space, said header being connected to a means for providing liquid foam, a liquid collection tray positioned to receive liquid passing through the wall of said liquid pervious outer foam barrier, said noise suppressor being mounted on a movable supporting structure. I

4. A noise sup ressor comprising: means for ra rally surrounding an unconfined noise-radiating stream of high-velocity gas with a blanket of liquid foam from a location adjacent the origin of said stream of gas for an axial distance at least equal to the diameter of said stream of gas at its origin.

5. A noise suppressor of claim 4 including perforate means defining an axial passageway, said passageway being adapted to receive said stream of gas, said passageway having a diameter at least twice as great as the diameter of the stream of highvelocity gas at the origin of said stream.

6. A noise suppressor of claim 4 including perforate means defining an axial passageway, said passageway being adapted to receive said stream of gas, and means for generating said blanket of liquid foam including means for regulating the size of the bubbles in said foam to predetermined values which are less than the size of the perforations in said perforate means.

7. A process of suppressing noise comprising:

radially surrounding an unconfined stream of high-velocity gas with a blanket of foamed liquid, said blanket extending axially rearward from a location adjacent the origin of the unconfined stream of gas for a distance at least as great as the diameter of the unconfined stream of gas at its origin, providing an axial passage for said stream of gas through said blanket of foamed liquid.

8. A process of claim 7 including regulating the size of the bubbles in said foamed liquid blanket to predetermined values.

9. A process of claim 7 wherein the unconfined stream of high-velocity gas is the exhaust from a jet engine and the inner diameter of the axial passageway in the blanket of foam is established at a value at least one and one-half times greater than the diameter of said exhaust at the point where said exhaust is ejected from said jet engine.

10. A process of suppressing noise according to claim 7 including continuously maintaining the blanket of foamed liquid radially around said unconfined stream of high-velocity gas for a predetermined period of time. 

1. A noise suppressor comprising: an outer foam barrier; an inner perforate foam barrier positioned wiThin said outer barrier and spaced from the wall of said outer foam barrier; a radially enclosed axially extending passageway defined by the inner wall of said inner barrier, said passageway extending axially completely through said noise suppressor; and means for substantially filling the space between said inner and outer barriers with foamed liquid.
 3. A noise suppressor of claim 1 wherein the outer foam barrier is a liquid pervious cylindrical member and the perforate inner foam barrier is a cylindrical member positioned concentrically within said outer foam barrier, the means for substantially filling the annular space between said inner and outer barriers with foamed liquid including a foam discharge header adapted to discharge liquid foam uniformly throughout said annular space, said header being connected to a means for providing liquid foam, a liquid collection tray positioned to receive liquid passing through the wall of said liquid pervious outer foam barrier, said noise suppressor being mounted on a movable supporting structure.
 4. A noise suppressor comprising: means for radially surrounding an unconfined noise-radiating stream of high-velocity gas with a blanket of liquid foam from a location adjacent the origin of said stream of gas for an axial distance at least equal to the diameter of said stream of gas at its origin.
 5. A noise suppressor of claim 4 including perforate means defining an axial passageway, said passageway being adapted to receive said stream of gas, said passageway having a diameter at least twice as great as the diameter of the stream of high-velocity gas at the origin of said stream.
 6. A noise suppressor of claim 4 including perforate means defining an axial passageway, said passageway being adapted to receive said stream of gas, and means for generating said blanket of liquid foam including means for regulating the size of the bubbles in said foam to predetermined values which are less than the size of the perforations in said perforate means.
 7. A process of suppressing noise comprising: radially surrounding an unconfined stream of high-velocity gas with a blanket of foamed liquid, said blanket extending axially rearward from a location adjacent the origin of the unconfined stream of gas for a distance at least as great as the diameter of the unconfined stream of gas at its origin, providing an axial passage for said stream of gas through said blanket of foamed liquid.
 8. A process of claim 7 including regulating the size of the bubbles in said foamed liquid blanket to predetermined values.
 9. A process of claim 7 wherein the unconfined stream of high-velocity gas is the exhaust from a jet engine and the inner diameter of the axial passageway in the blanket of foam is established at a value at least one and one-half times greater than the diameter of said exhaust at the point where said exhaust is ejected from said jet engine.
 10. A process of suppressing noise according to claim 7 including continuously maintaining the blanket of foamed liquid radially around said unconfined stream of high-velocity gas for a predetermined period of time.
 22. A noise suppressor of claim 1 wherein said inner and outer foam barriers are generally cylindrical and are mounted on a movable supporting structure. 